A segmented conical electric lens for optimization of the beam spot of the low-energy muon facility at PSI: a Geant4 simulation analysis

TL;DR

This paper presents a Geant4 simulation study of a segmented conical electrostatic lens designed to optimize the beam spot size and steering capabilities for low-energy muon experiments at PSI, supported by experimental validation.

Contribution

It introduces a novel conical electrostatic lens design and demonstrates its optimization for muon beam focusing and steering through detailed simulation and experimental comparison.

Findings

Optimized lens design reduces beam spot size.

Simulation results agree with experimental tests.

Enhanced beam steering capabilities achieved.

Abstract

The low-energy muon facility at PSI provides nearly fully polarized positive muons with tunable energies in the keV range to carry out muon spin rotation (LE-muSR) experiments with nanometer depth resolution on thin films, heterostructures, and near-surface regions. The low-energy muon beam is focused and transported to the sample by electrostatic lenses. In order to achieve a minimum beam spot size at the sample position and to enable the steering of the beam in the horizontal and vertical direction, a special electrostatic device has been implemented close to the sample position. It consists of a cylinder at ground potential followed by four conically shaped electrodes, which can be operated at different electric potential. In LE-muSR experiments, an electric field at the sample along the beam direction can be applied to accelerate/decelerate muons to different energies (0.5-30 keV). Additionally, a horizontal or vertical magnetic field can be superimposed for transverse or longitudinal field muSR experiments. The focusing properties of the conical lens in the presence of these additional electric and magnetic fields have been investigated and optimized by Geant4 simulations. Some experimental tests were also performed and show that the simulation well describes the experimental setup.